A DC electric starter-generator ("S/G") is typically used for bringing aircraft engines from a full-stop up to a rotational velocity sufficient to achieve engine ignition, and for subsequently generating electrical power. The S/G must be capable of producing high starting torque in order to turn an aircraft engine having substantial compression and inertial mass. In addition, the S/G must meet certain size and weight criteria in order to be acceptable for use on aircraft. While known aircraft S/G designs are capable of providing the necessary torque, they suffer from a number of significant drawbacks.
One major drawback is the collection of harmful, abrasive dust at points of rotation. Typically, known aircraft S/Gs include an armature assembly mounted on an armature shaft, the armature shaft being supported by bearings located at either side of the armature assembly. It is generally necessary to cool a S/G unit using a fan mounted to the armature shaft at one end, so that air is directed over the armature assembly and over a corresponding stator assembly whenever the S/G unit is in operation. In addition, it is common to commutate the armature windings by using commutation brushes. The commutation brushes generate dust particles which are directed downstream and which accumulate at the support bearings, causing the bearings to wear down prematurely. Typically, bearings must be replaced after every 1,000 hours of operation due to the dust collection problem. In addition, due to wear, the commutation brushes must be changed after every 400-600 hours of operation. This results in the need for frequent maintenance and service for known aircraft S/G designs.
Another major drawback of known aircraft S/G designs is their relatively low operating efficiency, typically hovering in the 70% range. Low efficiency S/Gs require higher current input, and take a longer time to achieve the necessary rotational velocity for engine ignition. This generally results in lower performance and possibly a shorter service life for the S/G and the aircraft engine, due to increased operating temperatures. A key factor which limits the performance of known S/Gs, both in terms of available starting torque and motor efficiency, is the limiting effect of counter electromotive forces (e.m.f.) produced in the armature--i.e. as the armature increases in rotational velocity and flux, the counter e.m.f. produced by the armature and acting on the stator field coils also increases, causing the current flowing in the stator field coils to be reduced, thereby decreasing the magnetic flux generated around the armature windings which produce the torque. The limiting effect of counter e.m.f. is inherent to all S/Gs, and known aircraft S/G designs have not been able to achieve efficiency much above the 70% range largely due to this problem. Other factors which decrease the efficiency of prior art aircraft S/G designs include friction introduced by commutation brushes, and high resistive copper losses in the series armature and stator windings.
Related to the problem of efficiency and service life is the issue of adequate cooling for the stator and armature assemblies and other components located within the S/G housing. In typical S/G designs, the air space between the armature and the stator is minimal. While the problem of air-cooling is minimized once the aircraft is airborne, the limited air space between armature and stator in present S/G designs limits the effectiveness of air-cooling while the aircraft is still on the ground. Consequently, a design which increases the air space and improves the airflow, while still being capable of producing an effective flux field, is desirable.